[cfe-dev] (not) initializing assembly outputs with -ftrivial-auto-var-init

[JF Bastien via cfe-dev]

Do the assembly routines set the constraints to indicate that they write to the input register? If so, we can do a simple dead store elimination change to notice that the assembly “call” changes the input. 

I think this will do what you want. 


> On Mar 21, 2019, at 5:35 AM, Alexander Potapenko <glider at google.com> wrote:
> 
> Hi JF et al.,
> 
> In the Linux kernel we often encounter the following pattern:
> 
> type op(...) {
>  type retval;
>  inline asm(... retval ...);
>  return retval;
> }
> 
> , which is used to implement low-level platform-dependent memory operations.
> 
> Some of these operations turn out to be very hot, so we probably don't
> want to initialize |retval| given that it's always initialized in the
> assembly.
> 
> However it's practically impossible to tell that a variable is being
> written to by the inline assembly, or figure out the size of that
> write.
> Perhaps we could speculatively treat every scalar output of an inline
> assembly routine as an initialized value (which is true for the Linux
> kernel, but I'm not sure about other users of inline assembly, e.g.
> video codecs).
> 
> WDYT?
> 
> 
> -- 
> Alexander Potapenko
> Software Engineer
> 
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